1. Field of the Invention
The present invention relates generally to an output sensing circuit for a circuit having a control loop, and in particular to an output sensor for a power supply having a main output and a switched output.
2. Description of the Related Art
For electrical power supplies, a standard configuration is that the power supply circuit has a main power output and a switched power output which is derived from the main power output. It is typically a problem to be able to remotely sense and regulate either of the outputs at any given time with a single remote sensing connection. For example, if the remote sensing connection is connected to the switched output and the switched output is either delayed in turning on at power up or is commanded to be turned off at any time by a logical signal, the feedback loop of the power supply will enter an open loop condition and cause the main output voltage to increase uncontrollably in magnitude, thereby causing possible damage to the load circuitry connected to the main output and possibly also to the power supply circuitry itself. On the other hand, if the remote sensing connection is connected to the main power output while the load circuit is connected to the switched output, the voltage at the switched output will not be tightly regulated due to a voltage drop associated with an electrical device, such as a MOSFET (Metal Oxide Semiconductor Field
Effect Transistor) which is used to provide the switched output. This voltage drop, which is dependent on the load current being drawn from the switched output, may have adverse effects on the load circuitry connected to the switched output.
Remote sensing is the term applied to the practice of closing the feedback loop of the power supply as close to the load as possible. The supply output voltage is sensed remotely. To make certain the feedback is always closed, a low ohmic value resistor is typically connected between the power supply output and the remote sensing input. If the remote sensing feature is utilized, the user connects the remote sense input to the output at the point where the tightest voltage regulation is to be maintained. This remote sense connection is much lower in resistance than the default resistance connecting the two nodes.
A problem with this approach is the inability of the remote sensing connection to withstand an inadvertent connection to the output return of the power supply. If this situation occurs, the low ohm resistor becomes connected across the entire power supply output and heavy current flows. The resistor usually fuses open due to excessive power dissipation. The power supply output will also uncontrollably increase-in magnitude due to the inadvertent connection.